Reductant dosing systems are typically used to reduce NOx emissions in large machines where space and weight considerations are not a concern, such as, for example, in locomotives and. stationary power generation applications. The reductant is stored in a tank located on the machine and, as the machine operates and produces exhaust, pumped from the tank into the machine's exhaust system. The reductant reacts with exhaust at high temperatures to affect a selective catalytic reduction (SCR) of NOx within the exhaust.
In order to comply with governmental exhaust regulations, precise control of reductant dosing may be required. This precision can be affected by air inside the dosing system. In particular, if air becomes trapped inside passages, valves, and/or the reductant pump of the dosing system, reductant may be displaced by the air. When this happens, an actual amount of reductant injected into the exhaust may be less than an expected amount. In addition, because air is compressible, the trapped air may act as a spring that absorbs the pumping action of the system, thereby making injection unpredictable, intermittent, or even impossible.
Another problem associated with conventional reductant dosing systems involves contamination of the system. Contamination can be caused by overheating or overcooling of the reductant during operation in extreme conditions. For example, overheating can cause the reductant to gel, while overcooling can cause formation of ice crystals. Both of these conditions can result in restrictions or clogging of dosing system components. When dosing system components are restricted or clogged, dosing precision can be reduced.
One way to improve precision in reductant dosing is disclosed in U.S. Patent Application Publication No. 2014/0352280 of Qi et al. that published on Dec. 4, 2014 (the '280 publication). Specifically, the '280 publication discloses a dosing system having a priming control state and a purging control state that are used to remove trapped air and contaminates from the dosing system.
During the priming control state, a pressure pump tank (PPT) is filled with compressed air via a first air valve, establishing a certain pressure Pc therein. A return valve is then opened, allowing the compressed air to push reductant out of the PPT and back into a reductant tank. When the PPT is empty, compressed air in the PPT will flow into the reductant tank, and a sudden change in pressure inside the PPT will be observed. Upon observing the sudden change in pressure in the PPT, the volume of reductant inside the PPT is determined to be zero and the return valve is closed. Thereafter, the first air valve is adjusted to set the pressure in the PPT to a desired pressure P1, which is lower than the pressure Pc. The first air valve is then closed, and a second air valve is then opened to establish the pressure Pc inside a liquid, supply tank (LST). The pressure gradient between the LST and the PPT causes reductant liquid to flow from the LST into the PPT. By measuring a pressure change in the PPT at this time, the liquid level in the PPT can be calculated. When the liquid volume reaches a value Vh, the second air valve is closed and the LST is allowed to vent. At this same time, the return valve is re-opened for a period of time to release trapped air in passages connecting the PPT to a dosing injector and in return passages and the return valve.
During the purging control state, the first air valve is closed, the second air valve is opened, and the return valve is opened. The resulting pressure of compressed air in the LST pushes reductant from the LST through the PPT and back into the reductant tank via the return valve. When the path from the PPT to the reductant tank is empty, a pressure drop will be detected. When this happens, the return valve is closed, thereby trapping air in the path, in the PPT, and in the LST. To clean the injector, after the return valve is closed and the compressed air is trapped in the path, the injector can be opened for a short period of time allowing remains from the injector to be released into an exhaust pipe.
Although the dosing system of the '280 publication may help to improve dosing precision by implementing priming and purging, the system may still be less than optimal. For example, re-opening the return valve for a period of time to release trapped air may not always provide desired results. Specifically, the period of time could change under different circumstances, leading to either excessive priming that is wasteful or insufficient priming that allows trapped an to remain inside the dosing system. In addition, purging based on any degree of pressure drop could be problematic. For example, it may be possible for one or more of the valves to be stuck in a partially closed position or for a passage to be partially obstructed. In this situation, a pressure drop could still occur, even though the pressure drop would not be the same pressure drop observed in a fully functional system, and the dosing system of the '280 publication could still consider the system to be purged.
The disclosed dosing system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art